Showing papers on "Relaxation (NMR) published in 1991"

Relaxation and retardation functions of the maxwell model with fractional derivatives

Abstract: A four-parameter Maxwell model is formulated with fractional derivatives of different orders of the stress and strain using the Riemann-Liouville definition. This model is used to determine the relaxation and retardation functions. The relaxation function was found in the time domain with the help of a power law series; a direct solution was used in the Laplace domain. The solution can be presented as a product of a power law term and the Mittag-Leffler function. The retardation function is determined via Laplace transformation and is solely a power law type. The investigation of the relaxation function shows that it is strongly monotonic. This explains why the model with fractional derivatives is consistent with thermodynamic principles. This type of rheological constitutive equation shows fluid behavior only in the case of a fractional derivative of the stress and a first order derivative of the strain. In all other cases the viscosity does not reach a stationary value. In a comparison with other relaxation functions like the exponential function or the Kohlrausch-Williams-Watts function, the investigated model has no terminal relaxation time. The time parameter of the fractional Maxwell model is determined by the intersection point of the short- and long-rime asymptotes of the relaxation function.

Dipole interactions with random anisotropy in a frozen ferrofluid.

Abstract: Glassy behavior (including hysteresis, irreversibility, a peak in the zero-field-cooled magnetization, and nonexponential relaxation) is observed in a quenched ferrofluid system consisting of 50-angstrom magnetite particles. An Arrott plot, M^2 vs H/M, shows clear features of random anisotropy similar to what is found in amorphous ferromagnets. We discuss the glassy behavior in terms of both the random anisotropy and the dipole interactions, and we contrast the unusual response of our system with canonical spin glasses.

Molecular dynamics and the phase transition in solid C 60

Abstract: We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with $^{13}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.

Cholesterol-induced fluid-phase immiscibility in membranes.

Abstract: The fluid-phase behavior of binary mixtures of cholesterol with phosphatidylcholines is investigated using magnetic resonance methods. Phospholipid biradicals provide the electron spin resonance spectroscopic resolution of two immiscible fluid phases in the dipalmitoylphosphatidylcholine-cholesterol system. Isotropic chemical shifts of the phospholipid carbonyl carbons in binary mixtures with cholesterol measured using solid-state high-resolution nuclear magnetic resonance methods furnish evidence for a putative hydrogen bond between the 3 beta-hydroxyl of cholesterol and the sn-2 carbonyl of the phospholipid. The location in the bilayer of cholesterol in the two fluid phases is determined by measuring spin label-enhanced spin-lattice relaxation rates of the 13C nuclei of both the phospholipid and cholesterol molecules. These results suggest, in a time-averaged sense, that in the cholesterol-poor fluid phase the cholesterol molecule essentially spans the bilayer, whereas in the cholesterol-rich fluid phase the molecule is present in both monolayers of the bilayer.

Quantitative 1H magnetization transfer imaging in vivo.

Abstract: A major factor contributing to proton (1H) spin-lattice relaxation in biological tissues is believed to be magnetization transfer between 1H in free bulk water and 1H restricted motion associated with macromolecules. We have shown recently that saturation transfer is an effective approach for studying this magnetization transfer process. Herein the determination of magnetization transfer rates in biological tissues is further analyzed by considering the time and power dependencies of saturation transfer. Following these analyses, quantitative magnetization transfer rate constant image maps were collected from the kidney in vivo. These rate constant images may prove useful in quantitative tissue characterization and in the determination of tissue-specific 1H relaxation mechanisms.

Rigorous link between fluid permeability, electrical conductivity, and relaxation times for transport in porous media

Abstract: A rigorous expression is derived that relates exactly the static fluid permeability k for flow through porous media to the electrical formation factor F (inverse of the dimensionless effective conductivity) and an effective length parameter L, i.e., k=L2/8F. This length parameter involves a certain average of the eigenvalues of the Stokes operator and reflects information about electrical and momentum transport. From the exact relation for k, a rigorous upper bound follows in terms of the principal viscous relation time Θ1 (proportional to the inverse of the smallest eigenvalue): k≤νΘ1/F, where ν is the kinematic viscosity. It is also demonstrated that νΘ1≤DT1, where T1 is the diffusion relaxation time for the analogous scalar diffusion problem and D is the diffusion coefficient. Therefore, one also has the alternative bound k≤DT1/F. The latter expression relates the fluid permeability on the one hand to purely diffusional parameters on the other. Finally, using the exact relation for the permeability, a ...

High frequency permittivity and its use in the investigation of solution properties

Abstract: Recent results from permittivity measurements at microwave to far-infrared frequencies are reported for various liquid electrolyte and non-electrolyte systems. They enlarge our knowledge on processes which produce (by their complex interplay of orientational, intramolecular, kinetic, H-bonding, diffusional and migrational modes) the properties of pure solvents, solvent mixtures and solutions. Protic solvents show three relaxational processes: re-establishment of t he perturbed solvent structure, "intramolecular" rotation of solvent molecules as monomers and in H-bonded chains or networks, and very short relaxa- tion times of about 1 ps due to H-bond dynamics. Aprotic solvents display a more or less continuous relaxation time distribution. Solvent mixtures show a particular behaviour related to the properties of their constituents. The addition of salt affects the relaxation times of t he solvents, but no new modes are generated by free ions. In contrast, ion pairs and other solute complexes act as dipoles and display specific relaxation processes.

Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the Staebler-Wronski effect

Abstract: Annealing and production of metastable defects in disordered solids is explained quantitatively with a model in which defect relaxation is a local phenomenon. The stretched-exponential time dependence of defect relaxation and the Meyer-Neldel rule for the relaxation-time constant are natural consequences of this model. The results are obtained by using an exponential distribution of activation barriers for transitions between the two states of the local defect. The model, applied to data in hydrogenated amorphous silicon, a-Si:H, gives an exponential distribution of barriers with a characteristic temperature of 220 \ifmmode^\circ\else\textdegree\fi{}C, roughly equal to the accepted freeze-in temperature for defect distributions in a-Si:H. The model explains that long degradation times convert defects with higher barriers and this results in longer annealing times. The microscopic models of the metastable defects in a-Si:H, weak-bond breaking and carrier trapping by charged dangling bonds, are discussed in the framework of this defect-relaxation model.

Transverse relaxivity of particulate MRI contrast media: from theories to experiments.

Abstract: Computer simulations and experimental approach have been used to characterize the properties of particulate MRI contrast agents with special attention paid to the influence of particle size. Up to approximately 50 nm, an increase of the particle diameter leads to a strong enhancement of the transverse magnetization decay rate. For larger grains or aggregates, the decay rate measured without spin-echo formation reaches a plateau. When observed through a spin-echo sequence, the transverse magnetization decay rate becomes slower on increasing the particle size or on shortening the echo time. For these large particles, multiexponential decay rates are observed. Definition and measurement of relaxivity in such systems is discussed. © 1991 Academic Press, Inc.

Structures of water and primary alcohol studied by microwave dielectric analyses

Abstract: By the use of the time domain reflectometry method dielectric measurements were carried out first on methanol mixtures with ethanol and 1‐propanol, and second, water mixtures with methanol, ethanol and 1‐propanol in the frequency range 10 MHz–20 GHz. The first mixtures show a Debye relaxation and logarithm of the relaxation time is given by a linear function of the mole fraction of methanol. These mixtures have the same chainlike cluster of pure alcohol. The second mixtures show the same trend of the relaxation time in a region 0≤xw 0.83 and that the cluster must be cyclic, consisting of six molecules.

Quantum efficiency and excited-state relaxation dynamics in neodymium-doped phosphate laser glasses

Abstract: Radiometrically calibrated spectroscopic techniques employing an integrating-sphere detection system have been used to determine the fluorescence quantum efficiencies for two commercially available Nd3+-doped phosphate laser glasses, LG-750 and LG-760. Quantum efficiencies and fluorescence lifetimes were measured for samples with various neodymium concentrations. It is shown that the effects of concentration quenching are accurately described when both resonant nonradiative excitation hopping (the Burshtein model) and annihilation by cross relaxation are accounted for by Forster–Dexter dipole–dipole energy-transfer theory. The Forster–Dexter critical range for nonradiative excitation hopping was found to be RDD = 11 A, while the critical range for cross relaxation was close to RDA = 4 A in these glasses. The quantum efficiency at low Nd3+ concentrations was (92 ± 5)%, implying a nonradiative relaxation rate of 210 ± 150 s−1 for isolated ions. Improved values for the radiative lifetimes and the stimulated emission cross sections for these glasses were also deduced from the measurements.

Scaling of the α relaxation in low-molecular-weight glass-forming liquids and polymers

Abstract: By measuring the complex dielectric susceptibility over 15 decades in frequency we compare the \ensuremath{\alpha} relaxation in low-molecular-weight glass-forming liquids and polymers. To describe the scaling behavior of the \ensuremath{\alpha} relaxation, a minimal set of four parameters is necessary, the mean relaxation time, the dielectric strength, and two shape parameters, describing the low- and high-frequency wings of the relaxation function, in pronounced contrast to a recently published scaling function.

Spin dynamics and electronic states of N- V centers in diamond by EPR and four-wave-mixing spectroscopy

Abstract: A new phase-modulation technique for nonlinear laser spectroscopy is applied with a relative resolving power in the sub-Hz range to measure fundamental relaxation processes of the N-V center in diamond. Complementary EPR experiments versus temperature establish the spin character of the ground state in the absence of optical illumination and show that spin-lattice decay occurs via two-phonon processes involving the dominant vibrational mode. The combined results permit deduction of reliable fine-structure assignments for three states of the center and accurate values for zero-field intersystem crossing and spin-lattice relaxation rates from linewidths of individual resonances in the four-wave-mixing spectrum.

Application of continuous relaxation time distributions to the fitting of data from model systmes and excised tissue

Abstract: Biological systems exhibit heterogeneity at many different levels, leading to the expectation of multiple relaxation time components for water protons in tissue samples. Traditional methods which fit the relaxation data to an a priori number of discrete components are open to observer bias in their interpretation of this data, and moreover, are intutively less realistic for heterogeneous systems than methods which produce continuous relaxation time distributions. Previous validations of continuous distribution techniques have been made on simulated data assuming uniform Gaussian noise. In the current work we have investigated the ability of one particular linear inverse theory technique to reproduce known relaxation time distributions from the data on a controllable model system. Furthermore, using the experience gained on the model system, we have applied this same technique to the analysis of in vitro relaxation time measurements on excised brain tissue and found for water protons in white matter, four reproducible components for the transverse relaxation, whereas gray matter gave rise to only two. The lougitudinal relaxation displayed only one component in either white matter or gray matter. © 1991 Academic Press, Inc.

13C NMR Spectroscopy of KxC60: Phase Separation, Molecular Dynamics, and Metallic Properties

Abstract: The results of 13C nuclear magnetic resonance (NMR) measurements on alkali fullerides KxC60 are reported. The NMR spectra demonstrate that material with 0 < x < 3 is in fact a two-phase system at e...

Dielectric relaxation of tert-butyl alcohol–water mixtures using a time-domain technique

Abstract: Dielectric relaxation measurements in the frequency range 10 MHz–10 GHz have been carried out in tert-butyl alcohol–water mixtures with various concentrations over the temperature range 273–313 K using a time-domain reflectometry (TDR) method. The bilinear calibration method as suggested by Cole has been used to correct the permittivity spectra. The corrected spectra could be fitted with a single relaxation time with a small amount of Cole–Davidson behaviour. Deviations from ideal mixing behaviour in the permittivity parameter (Iµo–Iµ∞) and relaxation time (τ) suggested the formation of a polymeric structure in tert-butyl alcohol–water mixtures. The dielectric relaxation behaviour showed the same structural changes as observed in ultrasonic relaxation. However, the maxima in excess permittivity and excess relaxation time occurred at different positions. This could not be explained by a simple model of the polymeric structure.

Quantitation of the susceptibility difference between trabecular bone and bone marrow: experimental studies.

Abstract: In this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient-echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T2, while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T2* as demonstrated in gradient-echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro. We have measured the MR relaxation times T1, T2, and T2* of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro, at 1.5 T, the relaxation times T1 and T2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/T2*, of saline in the presence of trabecular bone increases at a rate of 0.2 s-1/mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density-related T2* variations were observed for fat protons within the trabeculae where the chemical-shift-induced modulations of signal intensity in an oil-water emulsion have been separated from the susceptibility-induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T2* are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations.

From classical to quantum glass

Abstract: We study the effects of a transverse magnetic field on the dynamics of the randomly diluted, dipolar coupled, Ising magnet LiHo_(0.167)Y_(0.833)F_4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the introduction of quantum fluctuations.

Superconductivity and NMR investigations of KTl1.5-doped C60

Abstract: Doping of C60 fullerene with KTl1.5 results in superconductivity withTconset=17.6 K as shown by a dc magnetization measurement. Preliminary13C NMR studies on a doped and undoped sample are reported. Compared to original C60 the linewidth increases by a factor of 3, the resonance frequency is shifted by 42 ppm, and the relaxation rateT1−1 is an order of magnitude larger in the intercalated material at room temperature. Both observations support the picture of a metallic sample.

A 7Li nuclear magnetic resonance study of LiCF3SO3 complexed in poly(propylene‐glycol)

Abstract: 7Li nuclear magnetic resonance (NMR) linewidths and spin–lattice relaxation times for poly(propylene‐glycol) complexed with a range of concentrations of LiCF3SO3 are reported over the temperature region from 205 to 405 K. Calculations suggest that the spin–lattice relaxation mechanism is caused by the interaction between the 7Li (I=3/2) quadrupole moment and fluctuations in the surrounding electric field gradients, whereas the line shapes are influenced by both the dipolar and quadrupolar interactions. The motional parameters reported indicate that ion–polymer or ion–ion interactions are important in determining the Li+ cation mobilities. This is reflected in the lengthening of the correlation time with increase in Li+ ion concentration which suggests a decreased mobility for the cations resulting from a transient coordination of the cation to the polymer matrix or ion aggregation. Also, the activation energies in this study (∼0.24 eV) are in agreement with values obtained from recent pulsed field gradient studies suggesting that the NMR techniques employed in this study are approriate methods for probing the dynamics of ion transport on a macroscopic scale in these materials.

Lipid bilayer and water proton magnetization transfer: effect of cholesterol.

Abstract: Magnetization transfer between macromolecules and water can be a significant factor contributing to tissue water 1H relaxation. Using saturation transfer techniques, the degree of magnetization transfer between the macromolecular matrix and bulk water 1H can be directly measured and magnetization transfer contrast (MTC) can be generated in MR images. A significant degree of MTC has been observed in tissues with high plasma membrane content such as kidney and brain. The purpose of this study was to establish whether lipid bilayers, as models for cell membranes, could exchange magnetization with the water solvent and whether this effect could contribute to MTC observed in intact tissues. Magnetization transfer was measured in aqueous dispersions of egg phosphatidylcholine (EPC) in the presence and absence of cholesterol. It was found that neither EPC bilayers nor cholesterol by themselves significantly exchanged magnetization with bulk water 1H. However, as the concentration of cholesterol was increased, the pseudo-first-order magnetization exchange rate increased to a maximum value of approximately 1 s-1. The cholesterol-induced 1H magnetization exchange may be related either to longer correlation times of the lipid or to an increase in the number of water molecules associated with the bilayer. These results indicate that EPC-cholesterol bilayers exchange 1H magnetization with bulk water. These results are consistent with lipid bilayer contributions to bulk water relaxation and MTC in intact biological tissues.

Nuclear and Electron Relaxation: The Magnetic Nucleus-Unpaired Electron Coupling in Solution

Abstract: Part 1 Introduction: the concept of relaxation in molecular spectrosocpy magnetic resonance the spin Hamiltonian formalism hyperfine coupling effect of hyperfine coupling on relaxation statistical description of spin relaxation correlation time for the nucleus-electron coupling pulsed versus continuous-wave magnetic resonance. Part 2 Relaxation times: definition of a relaxation time at zero magnetic field definition of T1, T2, T1p the Bloch equations nuclear T1 and T2: and instructive picture chemical exchange as a source of relaxation relaxation of a system constituted by spin pairs the analogies between nuclear and electron relaxation. Part 3 Measurement of relaxation times and related experiments: experimental techniques for the measurement of nuclear longitudinal relaxation time T1 selective and non-selective nuclear T1 nuclear overhauser effect experiments experimental techniques for the measurement of nuclear transverse relaxation time T2 measurement of T1p - spin-locking expiments the field-cycling experiment, useful for determing and nuclear relaxation times at low magnetic field effect of short relaxation times on 2D NMR experiments experimental techniques for the measurement of electron relaxation times T1 and T2 the ENDOR experiment measurement of T1 and T2 (Nuclear-Electron Cross-Relaxation). Part 4 Electron Relaxation in dilute systems: physical picture of electron relaxation spin-orbit coupling electron relaxation mechanisms in the solid state - crystal vibrations, electron spin-phonon coupling electron relaxation in solution some numerical values. Part 5 Nuclear relaxation in paramagnetic systems: mechanisms of nuclear relaxation through coupling with unpaired electrons dipolar relaxation a pictorial description of dipolar relaxation diffusion-controlled dipolar relaxation contact relaxation Curie spin relaxation effects of splitting the S Manifold at Zero magnetic field - isotropic hyperfine coupling with the metal nucleus, anisotopic hyperfine coup oing with the metal nucleus g-Anisotropy, zero field splitting Redfield limit and beyond. Part 6 Electron and nuclear relaxation through NMRD: what is NMRD? copper(II) systems cobalt(II) systems nickel(II) systems manganese(II) systems other metal ions field dependence of . Part 7 Magnetic couples systems: effect of magnetic coupling on the electron relaxation times - unlike spin pairs, like spin pairs NMR parameters in magnetic coupled systems - isotropic shift, part contents.

Relaxation mechanisms in a benzyl chloride–toluene glass

Abstract: Using frequency-dependent dielectric susceptibility, we have studied three different types of relaxation phenomena, namely primary (\ensuremath{\alpha}), secondary (\ensuremath{\beta}), and conductivity (c) relaxation, in a sample of 25 vol % benzyl chloride in toluene. The measurement covers ten decades of frequency: ${10}^{\mathrm{\ensuremath{-}}3}$${10}^{7}$ Hz. The conductivity relaxation which is due to mobile ionic impurities in the sample has characteristics similar to those of the primary relaxation. Using the universal scaling curve for the primary relaxation response in glasses, we can separate the primary and secondary relaxations which overlap in frequency. The shape of the secondary relaxation is log-normal in the frequency domain and corresponds to a Gaussian distribution of energy barriers. The relaxation time for this process can be fitted by an Arrhenius form. Extrapolating the data to higher temperatures, we find that it crosses the primary-relaxation curve. We compare a set of similar molecular liquids and conclude that the secondary relaxation is mainly due to the rotation of a subgroup in the benzyl chloride molecule. We also report a measurement of the nonlinear dielectric response. There is no evidence of a divergent nonlinear susceptibility as the glass transition is approached.

"Spin"-flip scattering of holes in semiconductor quantum wells.

Abstract: We report results of calculations on the ``spin''-flip relaxation time of holes in semiconductor quantum wells due to hole interaction with static scatterers such as ionized impurities, alloy fluctuations, and s-d exchange (in the case of quantum wells based on diluted magnetic semiconductors). We show that size quantization along the growth axis leads to a drastic quenching of the ``spin''-flip scattering. This results in hole ``spin''-flip relaxation times, which can be much longer than the recombination time when the hole in-plane kinetic energy is small compared with the ${\mathrm{HH}}_{1}$-${\mathrm{LH}}_{1}$ separation distance.

Coherent-incoherent transition and relaxation in condensed-phase tunneling systems.

Abstract: The tunneling dynamics of the spin-boson problem has been computed using discretized path-integral simulations for temperatures T and couplings, i.e., the Kondo parameter \ensuremath{\alpha}, spanning the entire T-\ensuremath{\alpha} plane. The inherent problem of alternating weights has been solved using a combination of the stationary-phase Monte Carlo method and contour-distortion techniques. A transition from coherent to purely incoherent relaxation was observed for the spin correlation function. The time correlation functions and the location of the coherent-incoherent boundary on the T-\ensuremath{\alpha} plane are well described by the noninteracting-blip approximation. In the deep-tunneling limit of large \ensuremath{\alpha}, low T, and high bath frequency, the system relaxes exponentially, with its relaxation time constant following a power-law temperature dependence, in accord with perturbation theory. At higher T and low bath frequency, the relaxation time crosses over to a classical Arrhenius temperature dependence, reflecting the onset of activated processes. For a narrow region within 1/21, numerical results suggest that the system undergoes incoherent relaxation, with a short-time exponential decay, followed by a long-time tail of the power-law type. The short-time exponential relaxation time follows a peculiar power-law temperature dependence, with the relaxation rate increasing as a function of decreasing temperature.

The molecular environment of intracellular sodium: 23Na NMR relaxation

Abstract: The comprehensive approach described in the accompanying paper is illustrated here with the 23Na signal of a concentrated solution of bovine serum albumin (BSA) in saline and the intracellular (Nai) 23Na resonance of a dense suspension of Na(+)-loaded yeast cells We use frequency shift reagents to discriminate the latter from the extracellular resonance We find that the Nai signal corresponds to that of an effective single population of Na+ ions exhibiting a single type c spectrum This is true despite the fact that the yeast protoplasm is too large and too compartmentalized for a given Na+ ion to sample its entirety on the relevant NMR timescale Our results show clearly that, in addition to the decay of transverse magnetization, the recovery of longitudinal magnetization is biexponential This is required for a type c spectrum but has not often been detected The temperature dependence of the relaxation rate constants of the Nai resonance is not consistent with either a simple Debye process or a discrete exchange mechanism connecting two sites in the fast limit We have fitted the data using an asymmetric continuous distribution of correlation times for the fluctuations of electric field gradients sensed by the Nai nuclei The analogous distribution function for the Na+ in a 44% (w/w) BSA solution is quite similar to that of the Nai at the same temperature This suggests that while the macromolecular environment of the Nai ions is quite congested, it is also isotropic on quite a small spatial scale Also, one can use the correlation time distribution function, obtained from fitting the relaxation data, to calculate a relaxometry curve This is useful because experimental 23Na relaxometry is difficult The calculated curve may be a reasonable model for the mostly extracellular 23Na resonance encountered in vivo